Method and apparatus for fabricating and connecting a semiconductor power switching device

ABSTRACT

Fabrication processes for manufacturing and connecting a semiconductor switching device are disclosed, including an embodiment for dicing a wafer into individual circuit die by sawing the interface between adjacent die with a saw blade that has an angled configuration across its width, preferably in a generally V-shape so that the adjacent die are severed from one another while simultaneously providing a beveled surface on the sides of each separated die. Another embodiment relates to the manner in which damage to a beveled side surface of the individual die can be smoothed by a chemical etching process. Another embodiment relates to the manner in which the device can be easily mounted on a printed circuit board by providing conductive lands on the printed circuit board that are coextensive with metallized electrodes on the device and which can be placed on the printed circuit board and soldered in place and a unique lead frame which can be soldered to another electrode metallization on the opposite side of the chip and the printed circuit board in a manner which substantially reduces if not eliminates harmful thermal stress and which assures secure bonding notwithstanding elevation differences between the electrode metallization and the printed circuit board the lead frame is attached to. Another embodiment relates to an output connector for interconnecting an exciter circuit product with a spark producing device wherein the output connector utilizes a configuration that includes a sealing structure that is reliable and easily installed.

This application is a divisional of application Ser. No. 10/975,830,filed Oct. 28, 2004 now U.S. Pat. No. 7,144,792.

CROSS REFERENCE TO RELATED APPLICATIONS

A Solid State Turbine Engine Ignition Exciter Having ElevatedTemperature Operational Capability, filed Jun. 15, 2004, Ser. No.10/868,621, by Theodore S. Wilmot, John C. Driscoll and Richard S.Brzostek.

BACKGROUND OF THE INVENTION

The present invention generally relates to semiconductor fabrication andconnection methods and apparatus, and more particularly to fabricationand connection methods and apparatus high voltage semiconductor powerswitching devices that are useful in ignition exciters for turbineengine applications.

Modern turbine engine ignition exciters, especially those used in smallgas turbine applications, have evolved considerably in recent years;migrating from spark gap (plasma) switching devices and simplerelaxation type oscillator charge pumps to more reliable and predictablesolid state switching devices with digitally controlled DC-DC convertercharge pumps. Thermal performance of current art solid state ignitionexciters has been limited due to available thyristor switchingtechnology. Some designs use multiple series stacked phase controlthyristors with saturable reactors, while others employ switchingdevices specifically designed for pulse power applications. However,performance of both suffer from leakage current related limitations ofthe switching devices. At elevated temperatures, leakage current withinthe switching device results in increased power dissipation. Thiscondition precipitates additional leakage current, resulting in athermal runaway condition and device failure.

The most advanced current art exciters employ pulse type thyristors toeliminate the need for saturable magnetic components in the output stageand the associated limitations of that technology. While a considerableimprovement over phase control based designs, thermal performance ofcurrent art pulse thyristor based ignition exciters is still limited byswitching device leakage current. Moreover, current art technologyincorporates costly semiconductor die manufacturing and device packagingtechniques limiting commercial viability of the ignition exciters.

The ignition exciter embodiments in the above referenced Wilmot et al.application utilize a semiconductor pulse switching device that isdesigned to exhibit low leakage current at elevated ambienttemperatures. It has a design configuration that is disclosed in theWilmot et al application and in above referenced Driscoll et al. patentapplication. The design configuration also facilitates the use of novelfabrication and mounting processes and apparatus which greatly reducesthe cost of manufacturing and connecting the semiconductor pulseswitching device in an exciter product.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention relate to fabricationprocesses for manufacturing a semiconductor switching device. Oneembodiment relates to the manner in which a semiconductor chip isfabricated with regard to the separation of a plurality of individualcircuit die that are formed on a wafer, wherein the wafer is diced intoindividual circuit die by sawing the interface between adjacent die witha saw blade that has an angled configuration across its width,preferably in a generally V-shape so that the adjacent die are severedfrom one another while simultaneously providing a beveled surface on thesides of each separated die.

Another embodiment of the present invention relates to the manner inwhich damage to the beveled side surface of the individual die can besmoothed by an etching process.

Still another embodiment of the present invention relates to the mannerin which the device can be easily mounted on a printed circuit board byproviding conductive lands on the printed circuit board that arecoextensive with metallized electrodes on the device and which can beplaced on the printed circuit board and soldered in place.

Still another embodiment relates to a unique lead frame which can besoldered to another electrode metallization on the opposite side of thechip and the printed circuit board in a manner which substantiallyreduces if not eliminates harmful thermal stress and which assuressecure bonding notwithstanding elevation differences between theelectrode metallization and the printed circuit board the lead frame isattached to.

Still another embodiment of the present invention relates to an outputconnector for interconnecting an exciter circuit product with a sparkproducing device wherein the output connector utilizes a configurationthat includes a sealing structure that is reliable and easily installed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view that illustrates a power switching device (PSD)die configuration and gate and cathode metallization geometry;

FIG. 2 is a cross-sectional view with portions removed taken generallyalong the line 2-2 of FIG. 1 and illustrating the bevel geometry of oneembodiment of a PSD die configuration;

FIG. 3 is another cross-sectional view with portions removed similar tothat shown in FIG. 2, but illustrating a device having sides with adouble bevel geometry;

FIG. 4 is an idealized perspective view of a wafer having a plurality ofindividual die formed therein and illustrating a diagrammatic saw bladefor separating the individual die and simultaneously producing a chiphaving a beveled side geometry;

FIG. 5 is a cross-sectional view of a portion of a circular abrasive sawblade having an angled configuration across its width to dice a waferand produce the devices with a single bevel as shown in FIG. 2;

FIG. 6 is a cross-sectional view of a portion of a circular abrasive sawblade that has a double bevel angle V-shape configuration for producingthe die having the double bevel sides as shown in FIG. 3;

FIG. 7 is a diagrammatic plan view of the PSD shown from the bottomthereof illustrating the single bevel sides as shown in FIG. 2;

FIG. 8 is a microphotograph of a portion of an actual PSD illustratingthe topography of the sides after dicing with a saw blade having theconfiguration shown in FIG. 5. The microphotograph illustrating thedevice has a magnification factor of 1500× and illustrates the contrastin surface quality between dicing damage and etch removal.

FIG. 9 is a microphotograph of a portion of a chip similar to FIG. 8illustrating the texture of the sides after etching, the microphotographhaving a magnification factor of 130×.

FIG. 10 is a perspective view illustrating a PSD mounted to a printedcircuit board and particularly illustrating a lead frame whichinterconnects an electrode metallization to a land of the printedcircuit board;

FIG. 11 is a perspective view of an output connector for an excitercircuit mounted on a printed circuit board, the connector beingconnectable to a cable that extends to a spark generating device;

FIG. 12 is a plan view of the cylindrical insulator component of theoutput connector;

FIG. 13 is a side view of a cylindrical contact pin of the outputconnector;

FIG. 14 is a cross-section illustrating the contact pin installed in thecylindrical insulator;

FIG. 15 is a top view of the output connector with a potting materialenclosure applied to the pulse transformer assembly;

FIG. 16 is a front view of the potting material enclosure shown in FIG.15; and

FIG. 17 is a rear view of the output connector with a potting materialenclosure shown in FIG. 15.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the present invention relates to a semiconductor pulse switchingdevice that is particularly useful in implementing exciter circuitry forturbine engine ignition systems and particularly aircraft engineignition exciter circuits, the fabrication processes and techniques thatare described herein are applicable to fabricating and connecting othersemiconductor devices having other applications. However, the processesand apparatus that are disclosed herein are particularly configured foruse in high power circuitry where large currents may flow for shortperiods of time. Because high load transitory conditions may becommonplace for such devices, the fabrication and connecting processesand apparatus described herein may be useful across a wide variety ofapplications where high current flow may occur.

Referring to FIG. 1, an exemplary PSD switching device is illustratedgenerally at 20. The highly interdigitated gate-cathode structure ofthis device allows direct switching of high di/dt current pulses withoutthe need for protective (saturable reactor) networks. More particularly,a gate structure 22 is interdigitated with a cathode structure 24. Thegate structure 22 has two metalization pads 26 that are preferablysoldered to lands (not shown in this figure) of a printed circuit boardto which it is connected. The cathode structure 24 has a metalizationpad 28 that also facilitates connection with the printed circuit boardto which it is mounted. The compact die footprint (preferablyapproximately 0.38 inches square) and a thin base geometry improvesswitching speed by approximately 40% compared to prior art(approximately 0.5″ diameter) pulse thyristor devices having a circularconfiguration.

FIG. 2 illustrates a cross section of PSD switching device which has athickness that is preferably only about 0.024 inches, with the (I)region approximating 0.018 inches. As shown in FIG. 2, the sides of theswitching device 20 have a beveled surface which in FIG. 2 isapproximately 45°. Since the device is substantially square, the 45°bevel on sides 30 is achieved during the dicing process of fabrication.While the embodiment shown in FIG. 2 has a single bevel side of 45°, thedevice may be made with alternate (e.g., 57.40°) or multiple bevelangles such as is shown in FIG. 3. In this embodiment, a substantialportion of the device has a bevel angle of approximately 57° alongsurface 32 that merges with a bevel angle 34 that is much shallower andis preferably within the range of approximately 3 to approximately 10°relative to horizontal. While not particularly relevant to the manner inwhich the bevels are fabricated, it should be pointed out that thedouble bevel may be desirable when the operating voltage of the deviceis higher, i.e., substantially greater than 1850 volts and that thesingle bevel of approximately 45° is sufficient to operate in a reliablemanner. If the operating voltage is less than approximately 1850 VDC isutilized, it is possible to not have a bevel at all and the sides of thedevice may be perpendicular to the plane of the top and bottom surfacesof the device 20.

More significantly, the device 20 is preferably made using conventionaltechniques by producing a wafer 36 that is circular and includes aplurality of individual die 20. As is conventional, the wafer 36 isdiced into individual chips and in one embodiment of the presentinvention, a circular saw blade 38 is used to cut adjacent die from oneanother for further processing. This is done using saw blades that aremade to order and have a cross-sectional profile that is configured toproduce either a single bevel 30 as shown in FIG. 2 or a double bevel 32and 34 as shown in FIG. 3. A cross section of a saw blade that willseparate adjacent die at a uniform 45.0° angle is shown in FIG. 5 whichis configured to provide the 45.0° angle on both of the adjacent diesides 40 as well as to sever them from one another during the sawingprocess. In another embodiment, a straight cut (standard in theindustry) is used to separate the individual die, illustrated by the 90°to normal angle 33, while a secondary bevel cut is used to form theelectric field spreading bevel 35. Similarly, the configuration of theblade 42 has a double bevel configuration from both sides to the middlethereof which will produce the double bevel sides 32 and 34 as shown inFIG. 3. While the saw blades of FIGS. 5 and 6 are shown to have a singlepoint in the center thereof, it should be understood that the center maybe a relatively narrow point or it can be a wider pointed center portionas desired. In this regard, both FIGS. 2 and 3 have a slightly curvedportion 44 that returns toward the center of the device. Utilization ofa single bevel, double bevel, or a straight transverse cut for the sidesis dictated by the operating voltages of the chip and the electric fieldcharacteristics that may affect the operability of the device 20. Withregard to this embodiment of the present invention, the significantaspect of the present invention is that saw blades can have an outercross-sectional configuration to produce the appropriate single anglebevel, as well as a double angle bevel and potentially additional anglebevels as desired.

The saw blades are diamond grit; preferably approximately 17 micron gritsize. The blades are typically approximately 2 to 2.5″ diameter and areapproximately 0.06″ thick and are generally rotated at speeds ofapproximately 20,000 rpm. The blades 38 are generally made to order,such that alternate diameters and special thicknesses are possible.There is a standard hub size that is used in the industry.

A bottom view of the device 20 is shown in FIG. 7 in a diagrammaticalmanner and the upper right-hand corner is shown to illustrate a portionof a corner of two sides that are shown in the microphotographs of FIGS.8 and 9. With respect to FIG. 8, it illustrates at 1500 powermagnification the shape of two sides and an intersecting corner andillustrates the contrast between damage that is exhibited from thegrinding or sawing operation and the resulting surface after etching.Depending upon the voltage level of the circuit in which the device isbeing employed, the electric fields that are generated during operationmay affect the reliability of the device over time. With regard to theembodiment shown in FIG. 2 which has a 45.0° bevel on the sides 30, ithas been found that the damage that is exhibited to the side surface bythe operation of the saw blade can be substantially removed, if noteliminated by an etching process. In this regard, immersing the chips ina silicon etchant that is comprised principally of 33 parts by volumeNitric acid, 25 parts Hydrofluoric acid, 20 parts Acetic acid and 20parts Phosphoric acid, which results in a chemically etching operationthat effectively cleaves off damaged portions that are normal to the45.0° plane of the side of the device so that the resulting surface ismirror-like smooth as is shown in FIG. 9. Alternately, in yet anotherembodiment, if the silicon crystal has a 1,0,0 crystal orientation, aKOH (Potassium Hydroxide) etching process can be used to similarlysmooth damage normal to a 57.4° bevel angle. It has also been found thatthe chemical etching process is preferably conducted while usingultrasonic agitation. This is accomplished by gently agitating the chipsat a relatively low rate of about 10 to about 100 Hz while in theultrasonic bath, which prevents the formation of bubbles on the surfaceof the chips.

In accordance with another embodiment of the present invention, thedevice 20 having its beveled sides 30 can be mounted on a printedcircuit board 50 without having been encapsulated beforehand. The device20 preferably has its cathode and gate metalizations 26 and 28 locatedon the side that is to be placed in contact with the printed circuitboard 50 and suitable lands (not shown) are positioned to be coextensivewith the metalizations 26 and 28 of the device 20. A film of solderpaste is preferably placed between the device 20 and the printed circuitboard 50 on the metalizations 26, 28 of the device 20 can be presseddown upon the solder during installation. While this will connect thegate and cathode to the printed circuit board circuitry, the anodemetallization 52 is on the upper side of the device 20. To connect theanode metalization 52 to the printed circuit board, a lead frame,indicated generally at 54, is provided for interconnecting the anode toa land on the printed circuit board 50. To accomplish thisinterconnection, a film of solder paste 52 is placed between the anodemetallization 20 and a left end portion 56 and between a land 55 on theprinted circuit board and an opposite end 58 of the lead frame 54.

The lead frame 54 has an intermediate portion 60 between the endportions 56 and 58 and the intermediate portion 60 has a relativelylarge bend that extends between the chip 20 and the land on the printedcircuit board 50. The relatively large bend 60 is provided to reducethermally induced forces that may be developed due to various componentCTE (Coefficient of Thermal Expansion) mismatches that occur in thewidely varying ambient temperatures encountered in the harsh turbineengine environment. The lead frame is preferably manufactured of copperand has a thickness sufficient to conduct up to about 1500 amps ofcurrent for short periods of time during pulsing of the igniter when inan exciter circuit operation. Since the top surface of the device 20 maybe at a different elevation relative to the surface of the printedcircuit board where the end 58 is to be connected, both the ends 56 and58 are bent in a direction transverse to the longitudinal direction ofthe lead frame and both are in the shape of a shallow V to minimizeforces applied to the chip 20 during the solder reflow process. That is,as the solder solidifies, the V bends minimize rotational and normalforces applied to the chip and maximize alignment of the chip gate andcathode pads over their attendant PCB lands while likewise minimizingvariability of the gate and cathode interconnect solder column heights.

In accordance with another embodiment of the present invention andreferring to FIGS. 11-17, an output connector, indicated generally at66, interconnects a pulse transformer assembly (not shown in detail) toan igniter plug in a turbine engine ignition exciter system via a cable(not shown) that is connected to the output connector 66. Moreparticularly, the output connector 66 includes a connector bushing 68which is preferably made of aluminum and provides a protective outershield. The cable is inserted into the connector bushing 68 as well asinto an insulator 70 that surrounds a contact pin 72 that has anintermediate diameter portion 74 with an enlarged flange 76 that fitswithin the insulator 70. The outer surface of the insulator 70 has apair of annular grooves 78 which are configured to receive a pair ofO-rings 80 which operate to seal the outside of the insulator 70relative to the inside of the bushing 68. It has been found that the useof the O-rings, as contrasted with prior art methodology of merelyinjecting a silicon sealer between the two components, provides greatersealing reliability and is easier to install than injecting siliconebetween these two components. The insulator is preferably made of Ultem2300, a 30% glass filled polyetherimide formulation manufactured byGeneral Electric, which bring a higher level of heat resistance (400degrees F. range), excellent chemical resistance, high dielectricstrength, natural flame resistance, and extremely low smoke generation.The o-ring material is preferably fluorosilicone.

A potting compound is preferably used to provide an enclosure 82 aroundthe pulse transformer assembly and the connector bushing 68. Theenclosure 82 has a horizontal slot 84 which is filled by the printedcircuit board 50 and provides a sealed enclosure for the pulsetransformer assembly and the bushing 68. That coupled with the use ofthe O-rings 80 completely seal the contact pin 72 from extraneouselements. As shown in FIGS. 13 and 14, an aperture 86 is provided in theend flange 76 in a transverse direction relative to the insulator 70which has a slot 88 in line with the aperture 86. This permits an outputconductor from the pulse transfer assembly to be connected to thecontact pin 72 so that current can be conducted through the pin andcable to the igniter.

While various embodiments of the present invention have been shown anddescribed, it should be understood that other modifications,substitutions and alternatives are apparent to one of ordinary skill inthe art. Such modifications, substitutions and alternatives can be madewithout departing from the spirit and scope of the invention, whichshould be determined from the appended claims.

Various features of the invention are set forth in the following claims.

1. An output connector for a turbine engine ignition exciter circuit ofthe type which has a pulse transformer assembly and a printed circuitboard, said connector comprising: an outer hollow cylindrical connectorbushing for connection to a conducting cable sheath and cable connector;an intermediate hollow cylindrical insulator located within saidbushing, said insulator having at least one annular groove in the outersurface thereof; a cylindrical contact pin having an enlarged rear outerflange and a generally hemispherical front tip, said pin snugly fittingwithin a concentric cylindrical opening in said insulator with saidouter flange being at least adjacent to said insulator, the rear portionof said contact pin being configured to be electrically connected tosaid pulse transformer assembly and the front portion being configuredto engage a cable connector; at least one O-ring located within saidannular groove for sealing the interface between said insulator and saidconnector bushing against gaseous penetration.
 2. An output connector asdefined in claim 1 further comprising a second annular groove adjacentsaid one annular groove in said insulator and a second O-ring in saidsecond annular groove.
 3. An output connector as defined in claim 1wherein said outer flange abuts the rear end of said insulator.
 4. Anoutput connector as defined in claim 1 wherein said O-ring is made ofpreformed fluorosilicone material.
 5. An output connector as defined inclaim 4 wherein the fluorosilicone material forming said O-ring has asubstantially circular cross-section.
 6. An output connector as definediii claim 1 wherein said cylindrical insulator is made of a glass filledpolyetherimide material.
 7. An output connector as defined in claim 1further comprising an enclosure encompassing the pulse transformerassembly and at least a portion of said connector.
 8. An outputconnector as defined in claim 7 wherein said enclosure further comprisespotting material that is molded around the pulse transformer assemblyand at least a portion of said connector and is bonded to the printedcircuit board.